1. Field of the Invention
The present invention relates to a micro-abrading method and a micro-abrading tool and more particularly to a method for abrading a micro-region accurately in order to process a lens or an optical element with a high accuracy and an abrading tool to be used in carrying out the method.
2. Description of the Related Arts
It is necessary to abrade a nonspherical lens, an X-ray optical element, and the like incorporated in electronic equipment or an optical instrument. There is a demand for the development of a method for abrading a workpiece so that the workpiece has a configuration accuracy of as high as 0.01 .mu.m. To this end, it is necessary to abrade a very small area of the surface of the workpiece accurately from one area to the other.
Polishing processes or lapping processes have been adopted to abrade workpieces with high accuracy. But according to the conventional method, it is impossible to abrade the workpiece as accurately as less than 0.01 .mu.m. In order to overcome this problem, a magnetic abrading method using a magnetic abrading fluid as an abrading material has been developed recently. The term magnetic abrading fluid means magnetic fluid itself or an abrading material of fine particles suspended in the magnetic fluid.
According to the magnetic abrading method, magnetic abrading fluid is supplied between an abrading section positioned at the lower end of an abrading tool and a workpiece, and a magnetic field is formed therebetween. The magnetic abrading fluid is held between the abrading section and the workpiece according to a magnetic action with the magnetic abrading fluid applying pressure to the surface of the workpiece. When the abrading tool is rotated at a high speed in this condition, the magnetic abrading fluid is rotated at a high speed. As a result, magnetic abrading fluid abrades the surface of the workpiece. In this operation, in order to improve abrading efficiency, the direction and intensity of the magnetic field are changed to fluctuate pressure be applied by the magnetic abrading fluid to the surface of the workpiece and the motion of the magnetic abrading fluid is controlled. This magnetic abrading method is disclosed, for example, in Japanese Patent Laid-Open Publication No. 60-118466, Japanese Patent Laid-Open Publication No. 61-244457, and Examined Japanese Patent Publication No. 1-16623.
According to this magnetic abrading method, the magnetic holding force allows the abrading material to act concentratively on a very small area of the surface of the workpiece. Therefore, compared with the conventional abrading method, the workpiece can be abraded with a high accuracy. But even this magnetic abrading method is incapable of abrading the workpiece as accurate as less than 0.01 .mu.m.
That is, according to this method, since the magnetic abrading fluid is rotated at a very high speed by the abrading tool, abrading accuracy depends on the condition of the rotation of the abrading tool. When the abrading tool rotates, the number of rotations of the abrading tool fluctuates or a shaft deflection occurs. According to this method, the amount of abrasion fluctuates, the surface of the workpiece is locally abraded, and the area to be abraded fluctuates. It is necessary that a rotary mechanism has a spacial allowance so that each member makes a motion smoothly. Therefore, the motion of the abrading section is unstable more or less unstable and the surface of the workpiece is not uniformly and accurately abraded. That is, it is impossible to prevent the occurrence of the above problem when the abrading section rotates at a high speed.
According to the magnetic abrading method in which the abrading tool rotates at a high speed, the pressure for pressing the magnetic abrading fluid against the surface of the workpiece is not generated by the rotation of the abrading tool but by the application of the magnetic force, as described above. Therefore, unless the intensity of the magnetic force is high, a sufficient abrading force is not generated. The amount of abrasion of the workpiece changes with the fluctuation of the intensity of the magnetic force. This results in the magnetic force generating means, such as an electromagnet, being large and it is necessary to strictly control the intensity of the magnetic force. Further, in a magnetic circuit for obtaining the pressure for pressing the magnetic abrading fluid against the surface of the workpiece, it is necessary for the workpiece constitute a part of the magnetic circuit. Therefore, it is essential that the workpiece is magnetically conductive, namely, consists of a magnetic material. But if the workpiece is thin, a magnetic circuit can be constituted through the nonmagnetic materials. The pressure of the magnetic abrading fluid to be applied to the workpiece surface changes due to slight fluctuations in the thickness and magnetic properties of the workpiece. Therefore, it is difficult to adjust the abrasion amount and abrasion accuracy. Since the lens and the optical elements are non-magnetic and fairly thick, the magnetic abrading method in which an abrading tool rotates at a high speed cannot be applied to abrade them.
In order to solve the above problem, the present inventors developed a micro-abrading method and an abrading tool to be used in carrying out the method. According to the invention, actuators comprising piezoelectric elements are in motion in the XY-direction parallel with a workpiece surface and in the Z-direction perpendicular to the workpiece surface. The micromotion of the abrading section is transmitted to the magnetic abrading fluid so as to abrade the workpiece surface. Thus, the method and the tool therefor are capable of abrading the workpiece accurately, irrespective to the magnetic properties of a workpiece, and can be favorably applied to a workpiece comprising a non-magnetic material.
According to this method and the abrading tool therefor using the actuators comprising piezoelectric elements, it is unnecessary to rotate the abrading section at a high speed and constitute a workpiece as a part of a magnetic circuit provided to hold the magnetic abrading fluid and apply pressure to the magnetic abrading fluid. Therefore, this invention is capable of completely solving the above-described conventional problems.
That is, according to this method employing a novel driving system, the piezoelectric elements cause the magnetic abrading fluid to be pressed against the workpiece and move the abrading section by a slight amount along the surface of the workpiece. Thus, the workpiece is abraded. That is, the piezoelectric elements are used to, move the X-direction and Y-direction actuators and the Z-direction actuator of the abrading section by a slight amount so that the magnetic abrading fluid moves along the surface of the workpiece by a slight amount horizontally or vertically. The magnetic abrading fluid abrades the surface of the workpiece little by little due to its slight horizontal motion slight amount and applies pressure to the surface of the workpiece due to its slight vertical motion.
As described above, according to this method utilizing the novel driving system, it is unnecessary to rotate the abrading section. The actuators comprising piezoelectric elements include no sliding section or operating mechanism and are accurately driven according to an applied voltage. Thus, the magnetic abrading fluid undergoes a stable motion, which does not bring about unevenness or local abrasion of the workpiece surface. Since the amount of the actuator-operated motion of the magnetic abrading fluid in the XY-direction is smaller than that of a rotary motion of a shaft adopted conventionally, the workpiece can be finely abraded to a mirror-like surface finish with a high accuracy.
According to this method, since the magnetic abrading fluid is pressed against the workpiece by the piezoelectric elements constituting the Z-direction actuator, there is no need for providing a magnetic circuit connecting the abrading section of the abrading tool with the workpiece. Therefore, even though the workpiece is non-magnetic, it can be abraded in a manner similar to a magnetic workpiece. Since the pressure to be applied to the workpiece can be adjusted by appropriately setting the voltage of the actuator irrespective of the magnetic properties of the workpiece which often differ from each other due to material quality and the thickness thereof, abrading efficiency and abrading accuracy are not affected by the material quality and configuration of the workpiece. This method is capable of abrading a magnetic material such as steel which can be abraded by the conventional magnetic abrading method in which an abrading tool rotates at a high speed and, in addition, a non-magnetic material such as glass or ceramic which cannot be abraded by the conventional magnetic abrading method in which an abrading tool rotates at a high speed. Further, this method is capable of processing workpieces into a flat surface, a spherical surface, or a free-curved surface irrespective of the thickness thereof.
According to this abrading method employing the novel driving system, as described above, the slight motion of the abrading section is obtained by applying a voltage to piezoelectric elements. Accordingly, the magnetic abrading fluid is capable of abrading the workpiece surface uniformly and finely in a very small area in the vicinity of the abrading section. Compared with the conventional magnetic abrading method in which an abrading tool rotates at a high speed, the workpiece can be abraded uniformly and a very accurate mirror-like surface finish can be attained. That is, the workpiece can be abraded with a configuration accuracy of less than 0.01 .mu.m with ease and reliability.
This micro-abrading method and the abrading tool therefor can be applied to a method in which the magnetic abrading fluid is not used as the abrading material.
However, research made thereafter has revealed that this method has the following problems.
That is, according to the method utilizing this novel driving system, if the diameter of the abrading section is increased by enlarging the motion range thereof, the configuration of the abraded portion becomes non-uniform and a preferable spherical surface is not obtained, i.e., a smooth mirror-like surface finish cannot be obtained.
The reason the above problems occur is as follows:
According to the above-described micro-abrading method, the abrading section makes a cyclical motion, i.e., forms a circular arc-shaped Lissajous's figure about a supporting point according to the operational force of the XY-direction actuator which fluctuates cyclically. The workpiece surface is spherically abraded according to the motion of the abrading section. The abrading section is supported by the Z-direction actuator as well so that it is capable of moving not only in XY-direction, but also the Z-direction. But, the Z-direction actuator comprises piezoelectric elements which are not as rigid as the constructing material. Therefore, when the operation of the abrading section in the XY-direction becomes great, the supporting portion of the motion of the abrading section, namely, the Z-direction actuator, moves in the XY-direction. As a result, the motion locus of the abrading section becomes irregular and an accurate configuration of the circular-arc Lissajous's figure cannot be obtained.
FIG. 6 illustrates a sectional configuration of the motion locus T of the abrading section and an abraded surface H' formed on a workpiece W. The motion of the abrading section is determined by an excitation frequency applied to the X-direct-ion actuator and Y-direction actuator. Accordingly, the motion of the abrading section is expressed by the composition of an X-direction component X=A sin .omega.t and a Y-direction component Y=A sin (.omega.t+90.degree.). That is, the abrading section forms a circular arc of a radius A. In order to increase the radius of the abrading section, the amplitude of the excitation frequency to be applied to the XY-direction actuator is increased so that the radius A of the motion locus T becomes large.
However, the surface of the workpiece is abraded spherically with a center at the position of the abrading section according to the motion of the abrading section. As a result, the surface of the workpiece is concaved circularly along the circumference of radius A. That is, the abrading section does not pass the center of the abraded area H' and as a result, a portion Q remains unabraded and projecting in the center of an abraded area H. When the radius A is small, the projected portion Q is small and not outstanding. But if the radius A is large, the projected portion Q is outstanding. Hence, abrading accuracy is not favourable.